A Swiss-based Christian group says it has bought the freedom of more than 5,000 slaves in southern Sudan.

Christian Solidarity International (CSI) says it paid Arab middlemen $50 for each slave. It says the slaves, mostly Christian and animist women and children from the Dinka tribe of southern Sudan, have now been returned to their homeland.

CSI says the slaves were captured in raids by militia groups backed by the Islamic-led government as it fights a civil war with rebels from the mainly Christian south of the country.

But there is increasing disquiet about CSI's methods - the United Nations says the payments encourage more trafficking.

The BBC's East Africa correspondent, Martin Dawes, says that although there has always been slavery in Sudan, there is no doubt that Arab militias, as they go on their brutal raids, regard Africans as a crop to be harvested.

21,000 redeemed

Since CSI started buying back slaves in 1995 the organisation says it has redeemed nearly 21,000 people.

About $50 per person hs been paid in the past

Last year, our correspondent witnessed what was then a record figure of 410 people, mainly women, being exchanged for huge piles of Sudanese currency.

Now the organisation is said to have freed 5,500.

Because of the sensitivities of working in rebel-held southern Sudan, aid organisations will not talk openly of their concerns, but it is a common accusation that the figures are now too big to be credible.

There are allegations that senior figures within the rebel movement are rounding up villagers to falsely present to CSI in order to make money.

The head of the UN Children's Fund, Carol Bellamy, has said that buying back encourages more trafficking.

John Eibner of CSI admits that one can never rule out the possibility of fraud, but says he is convinced that the overwhelming majority have been enslaved and would be still in bondage or dead but for the work of his organisation.

Mr Eibner says the increase in numbers is because they are using more traders and it is a reflection of the large numbers of slaves still held in captivity.